Haplostachys haplostachya is an erect perennial herb. The stems are quadrangular, 30–60 cm long, and densely covered in white tomentose (woolly) hairs. The leaves are simple and opposite, with narrowly cordate (heart-shaped) blades measuring 8–17 cm long and 3–5.5 cm wide. The apex is obtuse to acute, and the upper surfaces are densely puberulent (covered in fine short hairs), green, and rugose (wrinkled). The lower surfaces are densely white tomentose. The blades are somewhat fleshy, with crenate (scalloped) margins. Petioles are 2.5–5 cm long, and stipules are absent. The flowers are subsessile, in simple, often dense, terminal racemes, 10–30 cm long. There are two flowers per verticillaster, and the bracts are oblong-lanceolate, usually 4–6 mm long, with the lowest pair reaching up to 20 mm long. The calyx is actinomorphic (radially symmetrical), cylindrical, 8–12 mm long, densely tomentose, with 5 linear teeth (2–)3–5 mm long. The calyx often splits longitudinally and is persistent. The corolla is zygomorphic (bilaterally symmetrical) and bilabiate, either white or sometimes tinged with purple externally. The tube is narrowly funnelform, somewhat curved, and ca. 14–22 mm long. The upper lip is 8–10 mm long, and the lower lip is 10–14 mm long with suborbicular lobes, the middle lobe being the largest with strongly undulate-crisped margins. A nectary disk is often present at the base of the ovary. There are 4 stamens, ascending under the upper lip, and included. The anthers are dithecal (having two pollen sacs). The fruits are nutlets that are 1-seeded, dark brown, and ca. 3.5–4 mm long, truncate at both ends, with an apex that is sparsely tomentose. These nutlets are enclosed in the persistent closed calyx, and the pericarp is thick and hard. Seeds lack endosperm or contain scanty oily endosperm. It was formerly known from barren ridges on Kauaʻi, the sandy isthmus and Kula on Maui, and Nohonaohae Cinder Cone, Puʻukapele, and Waikiʻi on Hawaiʻi Island. Presently, it is only known from Kipukakālawamauna on Hawaiʻi Island between 990–2,135 meters.